(1) Field of the Invention
The present invention relates to an extrusion die and a process for producing such an extrusion die. More particularly, the present invention relates to a honeycomb-shaped extrusion die adapted to extrude ceramic honeycomb structural bodies comprising body discharge channels and a plurality of independent body supply holes communicating with the body discharge channels as well as a process for producing the same. The extrusion die and the producing process thereof are characterized in that each of the body supply holes is so formed that the inner peripheral surface has different plural dimensions, thereby decreasing variations in the flow resistance of the body supply holes relative to a body passing therethrough.
(2) Related Art Statement
Ceramic honeycomb structural bodies have heretofore been used as catalyst carriers for purifying exhaust gases from internal combustion engines, fine particle-capturing filters, heat retainers, etc. The ceramic materials such as cordierite, alumina, silicon-carbide, mullite etc. There are known processes for producing the ceramic honeycomb structural bodies by extruding the ceramic material with use of an extrusion die.
For instance, conventional extrusion dies shown in FIGS. 3(A) and 3(B) are known (see U.S. Pat. No. 4,373,895, U.S. Pat. No. 3,790,654 and Japanese patent publication No. 57-61,592).
The conventional example (extrusion die 1) shown in FIG. 3(A) comprises a plurality of body supply holes 2 through which a body fed under pressurizing by a body feeder (not shown is passed, body stay zones 3 communicating with the body supply holes 2, and body discharge channels 4 having an arrangement corresponding to that of ceramic honeycomb structural bodies to be extruded (hereafter briefly referred to as "honeycomb structural bodies").
FIG. 3(B) is a partial sectional view of another conventional example. This example of FIG. 3(B) comprises a plurality of body supply holes 2 and body discharge channels 4 directly communicating with the body supply holes 2.
As is the same with the conventional examples in FIGS. 3(a) and 3(B), it is generally necessary to make uniform a flow rate of the body passing through the respective body supply holes 2 so that high quality honeycomb structural bodies may be extruded. For this demand, there are also known extrusion dies (not shown) in which a plate of noodle hole (Japanese patent publication No. 59-53,844) or a rectifier plate (Japanese patent publication No. 59-46,763) is provided on the side of the body supply holes.
In general, the body supply holes of the above conventional extrusion dies have a straight cylindrical shape. They are bored by drills. However, since hard metals such as die steel are used as the extrusion dies, such boring has poor workability. Further, there is a possibility that chips produced in the boring enters between the drill and a workpiece to make the roughness of the inner peripheral surface of the body supply hole coarse. Thus, the surface roughness differs among the inner peripheral surfaces of the respective body supply holes.
As mentioned in the foregoing, uniformalized flow resistance of a plurality of the body supply holes is an important requirement to produce high quality honeycomb structural bodies. When the inner diameter and the depth of the body supply holes are made constant, the flow resistance depends upon the roughness of the inner peripheral surface of the body supply holes. In addition, when the body supply holes are straight as in the case of the above-mentioned extrusion dies, the surface roughness so largely influences the flow resistance because the body supply holes are relatively small. Therefore, there arises large variations in flow resistance among the body supply holes of the conventional extrusion die. As a result, there exists an undesirable problem that it is difficult to manufacture honeycomb structural bodies of a high quality.
In order to remove the above-mentioned problem, the following countermeasures have conventionally been taken: For instance, the roughness of the inner peripheral surface is improved by honing or remaining after the body supply holes are bored. When the depth of the body supply holes is great, the surface roughness becomes further ununiform. In order to make the surface roughness of the body supply holes uniform, a die is divided into two die units, and slits and supply holes are machined in one of the die units, while only supply holes are formed in the other die unit. Then, they are bonded together. However, there occurs a problem that a manufacturing cost of the extrusion dies rises due to increased working steps.